A new poll by Lord Ashcroft contradicts previous findings and shows that more than half of Scots believe Trident should be replaced.

The Trident Nuclear Submarine, HMS Victorious, on patrol off the west coast of Scotland. Photograph: Getty Images.

That the Scottish public is opposed to British retention of nuclear weapons, not least due to their location on the Clyde, has long been regarded as one of the safest assumptions of the independence debate. A recent poll by Scottish CND found that 60 per cent of Scots oppose "the UK Government buying a new nuclear weapons system to replace Trident", with only 14 per cent in favour. Unilateral disarmament, it seems, remains one of Alex Salmond's strongest cards.

But a new survey by Lord Ashcroft, the Tory donor turned prolific pollster, suggests a more complex picture. It found that more than half of Scots believe Trident should be replaced, either with an equally powerful system (20 per cent) or a cheaper but less powerful system (31 per cent). How to explain the discrepancy? In his commentary on ConservativeHome, Ashcroft points out that, unlike his survey, Scottish CND's poll failed to mention that Trident was coming to "the end of its useful life", creating the misleading impression that the UK government "wanted to spend billions on new nukes just for the hell of it." He also criticises the group's use of the figure £65bn, which includes the running costs for thirty years, a number three times greater than "the true replacement figure". This seems reasonable; polls asking how much the government should spend on the NHS or schools typically cite the annual cost, not the lifetime one. (Although to add to the complexity, the poll also found that only 37 per cent support the UK having nuclear weapons "in principle", with 48 per cent opposed.)

To further test Scottish public opinion, Ashcroft asked a question including the replacement figure ("The cost of replacing Trident has been estimated at £20 to £25 billion pounds, and, just to be sure no one would miss it, underlined it too), but found it made no significant difference to the outcome. In fact, the number of Scots in favour of replacing Trident marginally increases to 53 per cent, with 24 per cent supporting an equally powerful system and 29 per cent supporting a less powerful one.

By a slim majority, Scots also believe that Trident should continue to be based in Scotland (43-39 per cent) and, even in the event of independence, only half say that Britain should not be able to continue leasing the Faslane naval base.

This morning, the SNP has responded by stating that it is "extremely confident that a majority of people in Scotland want to get rid of Trident nuclear weapons" and by accusing Ashcroft of displaying exactly the bias he complains of. "His question only included the procurement cost of a Trident replacement, when independent research shows that its lifetime costs will be nearly £100 billion," the party notes.

But regardless of the SNP's objections, the poll shows how malleable public opinion is. Ashcroft's survey didn't (but could have) mentioned the jobs currently dependent on Trident (520), and the Better Together campaign certainly will. Whichever side succeeds in getting its figures accepted as the "true" or most significant ones is likely to triumph in this debate.